The chart on p. 87 of Guns, Germs, and Steel is meant to show you the main causes of the “broadest pattern of human history.” That is, it shows you why, in Diamond’s view, people from the Eurasian landmass were able to become so much stronger than people from places like the Americas and Australia.
On the left of the chart is a vertical line with “ultimate factors” at the top and “proximate factors” at the bottom. This means that a factor that is lower on the chart was caused by a factor above it on the chart (as long as there is an arrow connecting the factors). For example, “large, dense, sedentary, stratified societies” helped to bring about technology, political organization, writing, and epidemic diseases. The factors at the bottom of the chart (not in boxes) were the factors that directly allowed the Eurasians to dominate other societies.
So, this chart is saying that the Europeans dominated the others because they had horses, guns, steel swords, ocean-going ships, political organization, writing, and epidemic diseases. Why did they have these things? They had epidemic diseases because they had many domesticated plant and animal species. They had horses because they had many suitable wild species. The rest of the factors came about because they had “large, dense, sedentary, stratified societies.”
Why did they have “large, dense, sedentary, stratified societies?” This was because they had food surpluses and food storage. They had these things because they had many domesticated plant and animal species (which means that they were farmers). Ultimately, they were farming because they had many suitable wild species and because species could spread across their landmass, helped by their long east-west axis.
In this chart, then, every factor that is higher up on the page causes the factors that it is linked to by an arrow. The lowest factors explain why Europeans dominated other societies. This is a brief outline of Diamond’s major argument in Guns, Germs, and Steel.